According to conventional wisdom, end-pumped lasers in which the pump light is incident on the laser medium coaxially with the laser output are more efficient than side-pumped lasers in which the pump light is incident on the side of the laser medium.
A typical end-pumped laser consists of a solid-state laser medium (i.e., laser crystal) that is pumped with light produced by one or more semiconductor lasers. In comparison with solid-state lasers that are pumped by lamps, such a laser is more compact in size. Such lasers also exhibit higher power efficiency, longer useful life, and greater ease of operation and maintenance.
With reference to FIG. 6, a conventional end-pumped solid-state laser system, excited by a diode laser, is shown. The laser system comprises a pump laser 1 (e.g., semiconductor diode laser) that produces a pump light flux 2. The pump light flux 2 passes through a pump optical system 3 to a solid-state laser medium 4. The pump light flux 2 is incident upon the laser medium 4 coaxially with a laser beam 5 produced by the laser medium. The laser beam 5 is transmitted by a partially reflective mirror 6.
A pump end of the laser medium 4 (i.e., the left-hand surface in the figure) comprises a surficial coating 7. The coating 7 exhibits a high reflectivity to the laser beam 5 and is transmissive to the pump light flux 2. A coating 8 is also applied to an exit surface (i.e., the right-hand surface in the figure) of the laser medium 4. The coating 8 is transmissive to the laser beam 5. The partially reflective mirror 6 and the coating 7 define a resonant cavity.
Further with respect to FIG. 6, the pump optical system 3 includes condensing lenses 3a, 3b that converge the pump light flux 2 onto the laser medium 4 according to a so-called "lens method." Other known methods for delivering the pump light flux 2 to the laser medium 4 include a "proximity method" in which the pump laser 1 is situated close to the laser medium 4, and a "fiber method" in which the pump light flux is guided to the laser medium 4 using one or more optical fibers. In the fiber method, first fiber end(s) are situated adjacent the pumping laser and second fiber end(s) can be placed proximally to the laser medium 4. Alternatively, lenses can be used to direct the pump light flux 2 from the second end(s) of the fibers to the laser medium 4.
Conventional lasers using the lens method, proximity method, or fiber method have several limitations.
First, the emission region of a semiconductor pump laser is generally asymmetric, with typical dimensions of about 500 .mu.m.times.1 .mu.m. The divergence of the emission is similarly anisotropic. As a result, the pump laser tends to illuminate a similarly shaped region of the laser medium 4, thereby limiting the efficiency and quality of the laser beam 5 produced by the laser medium 4. For example, the laser beam 5 can operate in a TEM.sub.00 mode, and a dilated pump light flux 2 can include light that is incident to the laser medium 4 outside a region corresponding to the laser beam 5 (e.g., having a circular transverse profile) produced by the laser medium 4. In such an instance, the pump light flux 2 is not fully utilized for pumping, yielding a reduced pumping efficiency. Also, excitation of the laser medium 4 outside the area required for TEM.sub.00 -mode emission can cause the resulting laser beam 5 produced by the laser medium 4 to be a higher-order transverse mode.
If the pump light flux 2 is sufficiently condensed to illuminate only a portion of the laser medium 4 compared to the laser beam 5, then the flux density of the pumping light flux 2 is excessively large. Absorption by the laser medium 4 of a pump light flux 2 having an excessively high flux density results in localized absorption saturation of the laser medium 4, which reduces efficiency.
In addition, if the portion of the laser medium 4 illuminated by the pump light flux 2 is small (e.g., the illumination portion is long and narrow) even though the cross-sectional profile of the pump light flux remains intact, then heat distribution in the laser medium 4 is non-uniform. This causes an asymmetric thermal lensing effect that distorts the laser beam 5.
The foregoing problems have been addressed in several ways in the conventional art. According to one approach, the transverse profile of the pump-light flux is made circular using prisms or cylindrical lenses. This approach increases the complexity of the pumping optical system, increases the time and effort required for alignment, and increases pump-light energy losses by reflection, etc., at the surfaces of the additional prisms or lenses of the pumping optical system.
In another approach, fiber methods are exploited that utilize multiple fibers. A first set of fiber ends is bundled and placed so as to match the shape of the emissive surface of the pumping laser, and an opposing second set of fiber ends is bundled in a shape (e.g., circular) corresponding to the desired laser-mode profile. This approach tends to avoid the problems that arise in proximity methods and lens methods. However, with respect to conventional fiber methods, especially such methods in which the fiber ends are placed in close proximity to the pumping laser, the size of the first set of bundled fiber ends is determined by the thickness of each constituent fiber, and the bundled fiber ends often extend outside the region corresponding to the circularly shaped laser beam 5 on the laser medium 4. In such instances, problems arise that are similar to problems encountered with conventional lens systems.
Accordingly, when utilizing a conventional fiber system, pump light flux is typically passed through a lens after being guided by the fibers. In such a system, the transverse profile of light produced by the bundled second set of fiber ends can be enlarged or contracted by the lens to cause the pumping light flux to be exactly and fully within the region on the laser medium 4 corresponding to the transverse shape of the laser beam 5. Unfortunately, in such a system, the lens must be aligned very accurately with the fiber ends and with the laser medium to obtain maximal irradiation of the laser medium 4. Such alignment adjustments are troublesome to perform accurately.
In addition, in conventional laser systems, a portion of the pumping energy is typically dissipated as heat in the laser medium 4. To efficiently remove such heat, the laser medium 4 is typically mounted on a metallic or other heat-dissipating body having good thermal conductivity. However, because the heat-dissipating body is a separate component from the pumping optical system, the parts count, and thus the cost, are higher.